After the drilling of an oil or gas well is completed, it is often necessary or desirable to pump certain fluids into the well to obtain improved production. For example, acids are sometimes pumped into a well to dissolve certain solids which impair production. Formations are fractured by pumping liquid at high pressure into the well, and fractures are held open by solid particles, such as sand, which are pumped into the well. High pressure pumps are also required for pumping cement into wells to cement casing in place.
High pressure reciprocating pumps are usually used for pumping such materials into the wells. Such pumps usually have two or more cylinders, and may operate at pressures of 10,000 pounds per square inch (psi), or more, up to as high as 25,000 psi. In addition, they often operate at very high speeds, up to 1000 strokes per minute.
The combination of corrosive or abrasive materials, high pressure and high speed constitutes very severe operating conditions for the valves used in these pumps. These valves consist of replaceable poppet-type valve bodies which seat on replaceable seats in the pump fluid end. The valve body usually includes a metal disk which seats on the seat, and a resilient insert which sealingly engages the seat. The seat is pressed or screwed into the valve deck in the fluid end of the pump, and provides an axial flow passage for the pumped fluid. The seat includes a guide surface to receive a valve guide which may consist of a lower stem extending axially downwardly from the valve body and engages a guide surface provided by a web in the seat or a "crowfoot" type guide which engages the inner circumference of the seat.
In a conventional valve design, such as that shown in U.S. Pat. No. 3,324,880, for example, the valve seat has an upwardly facing annular frusto-conical seating face surrounding the flow passage, and the disc of the valve body has a corresponding downwardly facing annular frusto-conical seating surface which supports the valve on the seat when the valve is in closed position. A resilient insert, usually made of a material such as polyurethane, is carried on top of the disc of the valve, and has a downwardly facing annular frusto-conical sealing surface positioned to sealingly engage the outer portion of the frusto-conical seating face of the seat.
On each pressure stroke the pressure created by the pump's piston forces the valve upwardly to an open position. When the pressure stroke is completed, the valve is forced downwardly by pressure from one or more other cylinders, and in some cases by a spring on top of the valve, until the valve strikes the seat, and the insert sealingly engages the seat.
The high operating speed makes inertia of the valve an important factor, since it is important that the valve be closed when the suction stroke begins; otherwise, the pressure on top of the valve will slam it down onto the valve seat with a very high force. This creates a high stress in the valve body as well as high bearing loads on the valve body and the seat. As a result, wear of the seating surfaces is accelerated. Wear is further accelerated as solid particles such as sand are trapped between the seating surfaces. In addition, repeated blows on the seat deform the inner edge of the seating surface, pushing it inwardly. This makes it necessary to bevel the inside circumference of the seating area.
In conventional valve designs, the metal disc portion of the valve body is below the insert, and engages the radially inner portion of the seating face of the seat, with the insert on top of the disc, engaging the radially outer portion of the seating surface. Solid particles between the metal seating surfaces of these valves tend to prop these surfaces apart, creating a gap into which the resilient insert may be forced by the pressure on top of the valve. When the resilient material is forced between the seating surfaces of the valve and seat, the material is pinched and chewed, reducing its ability to maintain a seal and accelerating failure.
In addition to the wear resulting from the impact of the metal valve body on the seat and that caused by solid particles trapped between the seating surfaces, erosion by abrasive particles carried by the fluid flowing through the open valve causes wear to the seating surfaces.
Another problem encountered when the resilient insert is carried on top or on the periphery of the valve is that it is sometimes torn away by the force of the flowing fluid and pumped down into the well, where it can cause considerable damage due to blocking bit jets, perforating holes, and the like. Such blockage can make it necessary to rework a well, sometimes at a cost of hundreds of thousands of dollars. If it is not lost in this way, the outer edge of the insert may be bulged out, so that its diameter is increased so much that the valve can't be removed through the valve pot opening.
Because of the wear suffered by valves in this service, it is not uncommon for valves to wear out in from one to eight hours. Wear is so severe on the metal parts that it is not economical to try to recondition them. In order to try to avoid time-consuming, expensive shutdowns during treatment of a well, the usual practice is to open up the pump and replace all the valves every day before operations are started.